Angiographic estimation of atherosclerotic tissue thickness is important in determining atherosclerosis regression or progression by serial antiographic studies. This estimation requires the prediction of non-atherosclerotic arterial taper. In this study, normal arterial taper has been characterized by measurements on angiograms. A formula derived from the principle of minimum work has been found predictive of the change in arterial width due to branching on in vivo angiograms (rabbit aortas) and in postmortem angiograms (rabbit aortas and human coronary angiograms). The formula is being used to predict proximal atherosclerotic tissue thickness from distal normal seqments and normal branch diameters. The difference between the atherosclerotic lumen (angiographic edge) and the predicted non-atherosclerotic taper (normal taper) represents atherosclerotic tissue thickness. Measurements on postmorten human coronary angiograms have predicted atherosclerotic wall thickness by this approach. This measure of atherosclerotic thickness is being tested by serial angiography in vivo during atherosclerotic progression and regression in rabbits. All antiographic predictions of atherosclerotic tissue thickness are being correlated with histological measurements of atherosclerotic tissue thickness. This method will be valuable in assessing atherosclerotic regression or progression by serial angiography in tapering arteries.